Travelling-wave tube



Jan. 10, 19756 A. LERBS 2,730,648

TRAvELLING-WAVE TUBEv Filed Jan. 18, 1950 2 Sheets-Sheet l Jan. 10, 1956 A. LERBS TRAVELLING-WAVE TUBE Filed Jan. 18, 1950 2 Sheets-Sheet 2 becomes Y greater than l.

United States Patent() TRAVELLING-WAVE TUBE Alfred Lerbs, Paris, France, assignor to Compagnie Generale de Telegraphie Sans Fil, a corporation of France Application January 1s, 1950, serial No. 139,318 Claims priority, application France February 4, 1949 'l Claims. (Cl. S15-3.5)

' My invention relatesto travellingwave tubes provided with a transverse magnetic iield, and its object is to obtain simultaneouslya high eiciency and a high amplification in 'such tubes.

It is known that a tube of the type in question has a retardation line, of the shape of a helix for example, which is raised to a positive potential with respect to another electrode extending in a parallel direction. An electron beam emitted by a cathode is introduced into theY space Vbetween these two electrodes at a speed which is substantially equal to the speed of propagation of an electromagnetic wave in the retardation line, and a magnetic field is applied transversely to the directions of the beam: and the electrostatic ield.

' In the .known constructions of this type of tube, the magnetic iield is constant all along the retardation line, and the distance between the two electrodes is also constant.

This arrangement has the drawback that the optinum conditions of anode voltage and of the magnetic iield are not identical for the gain and for the eiiiciency, since the electrodes describe cycloidal trajectories in the space between the electrodes and the induction of 'the magnetic field vB passes througha critical value Ber at the instant when said trajectories begin to touch the retardation line. It is'known that a high gain requires the ratio to be nearly equal to 1, whereas the eiciency increases more and more as Ber My invention makes it possible to combine a high gain with afrhigh eiiciency, by making Ber variable along the retardation line.

' It is thus possible to choose at the input end of the retardation line a value of B such that y Ber is nearly equal to 1. The electrons thus pass very close to the retardation line and arev located in a region in which the high-frequency eld is as high in value as possible, thereby permitting of an efficient phase focusing. By advancing along the retardation line, the ratio BCI' can increase without impairing the amplification mechanism, since on the one hand the electrons are already focused at the beginning of the line, and on the other 2,730,648 Patented J an. 10,y 1956 lCC hand under dynamic conditions they automatically move closer to the retardation line and transfer their energy to the high-frequency fieldwhich, moreover, has already increased owing to the amplification as it becomes more remote from the input end.

The ratio Ber is given by the expression: i

B Bai aBtM/ in which d denotes the distance and V the voltage across the electrodes.

It can therefore be seen that this ratio can be varied either by increasing B or by increasing d, or by combining the variations of both these factors in a suitable manner. Consequently, in accordance with the present invention, it is essential that the product of the intensity of the mag netic iield B and of the distance between the conductor and the retardation line, i. e. the distance d, increases from the input end to the output end of the retardation line, whereby the tube has a high gain in the rst part of the propagation space adjacent to the input end and a high eiiiciency in the second part of the propagation space reaching to the output end of the tube.

Owing'to the fact that the speed of the electrons, which can be'expressed by:

also varies with d or B, it is necessary to reduce the speed of the wave to that of the beam along the line, which can be effected asis known by acting on the pitch or the dimensions of the retardation line.

The invention will be better understood by considering the accompanying drawings in which:

Figs. la and lb show an example of the application of the principle of the invention to a tube of straight shape, the application of said principle being effected by varying the magnetic induction;

Figs. 2a and 2b show an example of application to the same shape of tube by varying the distance between the electrodes, and

Fig. 3 shows an example of application to a tube of circular shape, also by'varying the distance between the electrodes.

Fig. 1a shows a transverse section and Fig. 1b a sec tional plan view of a iirst exampleof a tube to which the invention is applied. In these figures, H denotes the retardation line, for example of helical shape, provided with an input end Eand an output end O'which are coupled to the coaxial lines L. Z denotes the electrode which is parallel to H and which is raised to a negative potential with respect to the retardation line. K denotes the cathode and C theusual collector of the tube.

According to the invention the magnetic iield B increases along the retardation line in the direction ofthe beam, by giving to the poles N and S, as shown in Fig. lb, a shape which slopes towards the tube as the beam moves forwards. At the same time the helix H is given a pitch that progressively decreases in the direction of the beam, in order to decrease the speed ofthe wave as the speed of the beam decreases. The curve a represents the trajectory of an electron under static conditions and the curve b the trajectory of an electron of favorable phase, under dynamic conditions, which transfers energy to the high-frequency lield. It can be seen that such an electron always remains in the region of highpotential, that the amplitude of the cycloidal trajectory is very substantially equal, at the input end, to the distance between the electrodes, which lfavors efiicient arnp'litication, and that said amplitude decreases along the line, which favors a high efficiency.

ln the modification of Figs. 2a and 2b, which show similar sections to Figs. la and lb and in which the same reference letters denote identical members, the poles N and S, as shown in Fig. 2lb, are parallel to the tube as in the usual systems, but in this case the electrode Z is no longer parallel to H but, as shown in Fig. 2a, gradually becomes more remote therefrom in the direction of the beam. The curve b shows that the same eiect is obtained as in Fig. la, the curve a only differing from the corresponding curve of Fig. la by the invariable amplitude of the cycloid.

le efficiency in the cases of Figs. l and 2 is approximately the same, since it is given by the expression:

ggf ;l B and does not depend on the means by which the variation of t Bcf is etl'ected.

In order to obtain a high efficiency, the electrons must only reach ythe retardation line at a point which is as remote as possible from the input end, since in that case BCT and the eiiiciency have higher values. In order to prevent the retardation line from collecting too great a quantity of electrons, which would lead to heat losses,

the system must be so dimensioned that the electrons almost touch the retardation line near the output end and are collected after the output passage by a collector which can be readily cooled. This dimensioning can be easily eiected in combination with the means proposed by the invention.

` The invention can also be applied to a tube of circular shape. In this case, the solution involving a variable distance with a constant field is more favorable, since the production of a magnetic field that increases from the input end to the output end is rather ditlicult to obtain for the circular shape. Fig. 3, which uses the same reference letters as the previous figures, relates to this case. This figure does not require further explanation, since it only shows as a whole a travelling-wave tube ot' known circular shape provided with a transverse magnetic field, except that the inner electrode Z is bent into a spiral so as to become gradually more remote from the helix as the wave is propagated through the retardation line.

The invention is capable of being subjected to many modifications which do not alter its principle. Thus, the two solutions (variable field and variable distance) could be combined so as to obtain the most judicious law of variation of the ratio of the field to its critical value. In the variable field solution, the angle of inclination of the profile of the poles and the shape of said profile could be made suitable for correcting .the distribution of the field and reducing it to any desired law. The principle of the invention does not depend on the shape of the tube or on the construction of the retardation line.

What I claim is:

l. ln a travelling wave tube arrangement, in combination, an electron source emitting an electron beam along a predetermined path; a collector of electrons arranged spaced from said electron source in the path of said electron beam; a retardation line having an input end arranged in the region of said electron source and an output end arranged in the region of said collector of electrons, said retardation line extending along said predetermined path ot' said electron beam; means connected to said input end of said retardation line for exciting an electromagnetic wave therein; means connected to said output end of said retardation line for collecting the energy amplified by the electron beam; a conductor extending between said source and said collector and being spaced apart from said retardation line a distance of a predetermined magnitude so as to define therewith a space for the propagation of the electron beam emitted by said source; means for applying an electrostatic field between said retardation line and 'said conductor; and means for establishing a magnetic field having an intensity of predetermined magnitude in the space between said retardation line and said conductor at right angles to said electrostatic field and to the electron beam, one of said magnitudes gradually increasing from said input end to said output end of said retardation line, whereby the product of the intensity of said magnetic field and of said distance between said conductor and said retardation line gradually increases from said input end to said output end of said retardation line so that the tube has a high gain in the first part of the propagation space adjacent to said input end and a high efficiency in the second part of said propagation space adjacent to said output end. 1

2. in a travelling wave tube arrangement, in combination, an electron source emitting an electron beam along a predetermined path; a collector of electrons arranged spaced from said electron source in the path ofsaid electron beam; a retardation line having an input end arranged in the region of said electron source and an output end arranged in the region of said collector of electrons, said retardation line extending along said predetermined path of said electron beam; means connected to said input end of said retardation line for exciting an electromagnetic wave therein; means connected to said output end of said retardation line for collecting the energy amplified by the electron beam; a conductor extending between said source and said collector and being spaced apart from said retardation line so as to limit therewith a space for the propagation of the electron beam emitted by said source; means for applying an electric tield between said retardation line and said conductor; and means for establishing in the space between said retardation line and said conductor and at right angles to, the electric field and to the electron beam, a magnetic tield the magnitude of which gradually increases from said input end to said output end of said retardation line.

3. In a travelling wave tube arrangement, in combination, an electron source emitting an electron beam along a predetermined path; a collector of electrons arranged spaced from said electron source in the path of said electron beam; a retardation line having an input end arranged in the region of said collector of electrons, said retardation line extending along said predetermined path of said electron beam; means connected to said input end of said retardation line for exciting an electromagnetic wave therein; means connected to said output end of said retardation line for collecting the energy amplified by the electron beam; a conductor extending between said source and said collector and being spaced apart from said retardation line a constant distance so as to limit therewith a space for the propagation of the electron beam emitted by said source; means for applying an electric field between said retardation line and said conductor; and means for establishing in the space hetween said retardation line and said conductor and at right angles to the electric iield and to the electron beam, a magnetic field the magnitude of which gradually in'- creases from said input end to said output end of said retardation line, whereby the product of the intensity of said magnetic field and ot said distance between said conductor and said retardation line gradually increases from said input end to said output end of said retardation line so that the tube has a high gain in the first part of the Apropagation space adjacent to said input end and a high eiciency in the second part of said propagation space adjacent to` said output end.

4. In a travelling Wave tube arrangement, in combination, an electron source emitting an electron beam along a predetermined path; a collector of electrons arranged spaced from said electron source in the bath of said elec tron beam; a retardation line having an input end arranged in the region of said electron source and an output end arranged in the region of said collector of electrons, said retardation line extending along said predetermined path of said electron beam; means connected to said input end of said retardation line for exciting an electromag netic wave therein; means connected to said output end of said retardation line for collecting the energy amplified by the electron beam; a conductor extending along suhstantially the entire length of said retardation line and spaced therefrom a distance gradually increasing from the input end to the output end of said retardation line,

Vsaid conductor defining with said retardation line suba predetermined path; a collector of electrons arranged spaced from said electron source in the path of said electron beam; a retardation line having an input end arranged in the region of said electron source and an output end arranged in the region of said collector of electrons, said retardation line extending along said-predetermined path of said electron beam; means connected to said input end of said retardation line for exciting an electrof magnetic wave therein; means connected to said output end of said retardation line for collecting the energy amplifed by the electron beam; a conductor extending between said source and said collector and located along said retardation line spaced therefrom a distance gradually increasing from the input end to the output end of said retardation line, so as to limit with said retardation line a space for the propagation of the electron beam emitted by said source; means for applying an electric lield between said retardation line and said conductor; and means for establishing in the space between said retardation line and said conductor and at right angles to the electric field and to the electron beam, a magnetic field the magnitude of which gradually increases from said input end to said output end of said retardation line, whereby the product of the intensity of said magnetic field and of said distance between said conductor and said retardation line gradually increases from said input end to said output end of said retardation line so that the tube has a high gain in the rst part of the propagation space adjacent to said input end and a high etliciency in the second part of said propagation space adjacent to said output end.

6. In a traveling wave tube arrangement according to claim 2, said means for establishing a magnetic field including two pole pieces arranged on opposite sides of said retardation line and forming between themselves a gap the Width of which gradually decreases from said input end to said output end of said retardation line.

7. In a traveling wave tube arrangement according to claim 1, said retardation line consisting of a helix the pitch of which gradually decreases from said input end to said output end of said retardation line.

References Cited inthe file of this patent UNITED STATES PATENTS 2,241,976 Blewett et al May 13, 1941 2,409,992 Strobel Oct. 22, 1946 2,511,407 Kleen et al. June 13, 1950 2,531,972 Doehler et al Nov. 28, 1950 2,516,944 Barnett Aug. 1, 1950 2,617,961 Bruck Nov. 11, 1952 OTHER REFERENCES Pages 328-330, Annales de Radioelectricite, vol. 3, No. 14, October 1948. Article by I. Brossart and O. Doehler. Copy in Pat. Otr. Seien. Libr. 

